1. Field of the Invention
The present invention relates generally to coal shearing machines and more particularly to a method for controlling a coal shearing machine from a remote location using a medium frequency communications systems and coal-rock interface sensors.
2. Description of the Prior Art
For the past several years, supply in the coal mining industry has exceeded demand. This over-supply has led to an increase in the industry's competitive intensity which, in turn, has led to a heightened awareness by coal producers of the need to reduce cost and risk in mining operations. In opposition to the desire to cut costs and improve safety, is the problem that only deeper, thinner, lower quality and higher cost coal reserves are left to mine.
In an effort to aid the mining industry in resolving this dilemma, the Jet Propulsion Laboratory (JPL) conducted a study aimed at evaluating automated longwall mining technology. See W. Zimmerman, R. Aster, J. Harris and J. High, Automation of the Longwall Mining System, JPL Publication 82-99 (Nov. 1, 1982). Among other things, this study identifies the need for developing remote control technology for longwall shearer operations.
Remote control of the shearer requires short range sensing of the coal-rock interface in order to keep mine personnel out of the hazardous coal cutting zone (face). Continuous and longwall mining requires the operator to be in close proximity to the coal cutting edges (drum) so he can see the cutting horizon and keep the cutting edges from striking rock. In the process, the shearer operator is constantly in a hazardous area. If the shearer cutting edges are allowed to strike rock, flying sparks can cause methane and coal dust ignitions. Cutting into sandstone roof/floor produces silica in the dust which causes non-compliance with MSHA respirable dust regulations. In mining, the hazard is often alleviated by slowing down the tram rate of the shearer, cutting only in the direction of the face ventilation air stream, or increasing the water spray to disperse the dust plume. In addition to the dust problem, wear and tear on the cutting drum and bearings of the mechanical drive components often leads to increased down time and maintenance problems.
Another requirement for effectively automating a longwall mining system is the development of a reliable remote control communication system. Various longwall manufacturers in the U.S. and Europe currently offer VHF (very high frequency) and LF (low frequency) remote control systems. The LF system consists of a control link from the headgate command center to the shearer via the AC power cable. The LF system is limited since it does not allow remote control from a shearer operator anywhere along the face. VHF and UHF systems work well on line of sight signal propagation paths to control continuous mining equipment and roof bolters. The technology fails, however, in the remote control of trains in tunnels and loading panels such as are used in block cave mining. The reasons why the VHF and UHF systems fail to work in such situations are: VHF and UHF signals suffer great attenuation when propagating down the waveguide created by the shield and pan line, reliable control is limited to line-of-site operation, rolls along the face can limit control range, and the reflected signal energy from the longwall steel support members produce nulls in the transmitting waves. Because of the problems associated with VHF and UHF transmissions, the radio transmission signal in the "dead control" null zone will be below that required for a low bit error rate. This excessive bit rate results in command signals being improperly decoded or not responded to at all.
To enable the control of the shearer (or continuous miner) from a safe distance, various attempts have been made to develop coal-rock sensor technology. In Europe and the U.S., researchers have investigated natural radiation background sensor technology. Using the natural background radiation of the above strata, this system allows the coal thickness above the shearer to be measured and maintained as the shearer cuts; however, this sensor fails to reliably work in some geologies. Other similar applications of technology include the use of acoustics and the "sensitive pick" for seam thickness measurement and coal-rock interface detection, and the investigation of microwave measuring techniques by researchers at the National Bureau of Standards. The thrust of the natural radiation background sensor, acoustics and microwave measuring technologies was to increase the shearer operator's control capability so he could cut the maximum amount of coal possible with each pass.
Other sensors were developed to solve face alignment problems contributing to many conveyor and pan line failures. One of these sensors was the yaw measurement sensor developed by the Benton Corporation. This sensor measures angular deviations in the pan line and transmits information to a computer. The computer determines the position of the shearer and the straightness of the face conveyor. In a U.S. Government report, the NASA Marshall Space Flight Center Longwall Program tested the performance of several shearer and conveyor sensors and then examined design problems associated with retrofitting the shearer and conveyor with the most promising sensors.
Finally, Chang and Wait have disclosed a theoretical proposal for using a resonant loop antenna as a probe for the determination of roof thickness in a coal mine operation. See D. Chang and J. Wait, An Analysis of a Resonant Loop as an Electromagnetic Sensor of Coal Seam Thickness, Proceedings of URSI Conference on Remote Sensing, LaBaule, France (Apr. 28-May 6, 1977).